We recently looked at biosignatures as part of a discussion about using polarized light to examine exoplanet atmospheres. As if on cue, we now get a reminder of how carefully the biosignature hunt must proceed. It’s not enough, for example, to find one or two interesting gases in a distant atmosphere, for natural processes can account for potential biomarkers, which is why we need to find gases like ozone and methane, oxygen and carbon dioxide existing simultaneously. The most recent discovery from Cassini data puts an exclamation point on the matter with the discovery of molecular oxygen ions in the thin atmosphere of Dione, one of Saturn’s 62 moons.
With a radius of no more than 560 kilometers, Dione is evidently composed of a layer of water ice surrounding a rocky core. We are not, obviously, talking about a thick atmosphere around a world this small. Cassini and its CAPS instrument (Cassini Plasma Spectrometer) closed to within 503 kilometers of the surface in April of 2010, finding one oxygen ion for every 11 cubic centimeters of space in a gaseous envelope thin enough to be called an ‘exosphere.’
Image: The ragged surface of Saturn’s moon Dione. Credit: NASA/JPL-Caltech.
Robert Tokar (Los Alamos National Laboratory), lead author of the paper on this work, notes that the concentration of oxygen in Dione’s atmosphere is the equivalent of what we would find at an altitude of about 480 kilometers in Earth’s atmosphere. Adds Tokar:
“We now know that Dione, in addition to Saturn’s rings and the moon Rhea, is a source of oxygen molecules. This shows that molecular oxygen is actually common in the Saturn system and reinforces that it can come from a process that doesn’t involve life.”
The process is thought to work like this: During Dione’s 2.7-day orbit of Saturn, the moon is struck by charged particles produced by the planet’s inner magnetosphere, causing molecular oxygen ions to be displaced into the tenuous atmosphere, after which they are again stripped by the planet’s magnetosphere. The process of molecular oxygen displacement is called ‘sputtering,’ and while the paper notes some uncertainties in its calculations — surface temperature variations on Dione can be significant — it emphasizes the core finding:
…what is not uncertain is we report here the first in situ detection of a component of Dione’s thin sputter produced atmosphere by collecting the pick-up ions. Since the pick-up ion density is directly related to the atmospheric densities, we have also obtained a rough estimate of the atmospheric O2 density. This is consistent with the earlier observations of oxygen products trapped in the surface ice and places Dione in a category with Europa, Ganymede, Rhea and Saturn’s main rings all of which have oxygen atmospheres.
The exosphere around Rhea was detected in 2010 and is similar to Dione’s, with an oxygen density at the surface of some 5 trillion times less than what would be found at the Earth’s surface. Data from Cassini’s ion and neutral mass spectrometer from a later flyby are also under investigation. The Dione finding is not completely without interest for astrobiologists, given that molecular oxygen might be able to combine with carbon in the sub-surface lakes of gas giant moons like Europa.
The paper is Tokar et al., “Detection of exospheric O2+ at Saturn’s moon Dione,” Geophysical Research Letters Vol. 39 (2012), L03105 (abstract).
I was going to add a comment to your article on polarized light about this, but you beat me to it with this article. Either you are very quick or I’m slowing down in my old age… LOL. Good write up as usual.
Tom
Io has a thin atmosphere of sulfur and sodium…
By the way, I am of the opinion that the oxygen atmosphere of earth is PRIMARILY because of photolysis of water and ammonia in the high atmosphere releasing hydrogen , which then escapes, while the heavier oxygen and nitrogen remained trapped in the earths gravity well. while there has been some sequestration of reduced carbon( coal , oil and natural gas) it is not enough to account for all the O2 in the atmosphere.
The implication is that detection of an O2 atmosphere from an exo planet does NOT prove that the planet has life. Detecting a chlorophyll signature would work though.
Thin oxygen atmospheres seem to be a common feature of ice moons: Galileo detected a similar atmosphere at Europa and there is evidence for oxygen at Ganymede and Callisto as well, though this is less well established.
While this is interesting, such minor trace amounts of oxygen could not ever be confused with a breathable atmosphere. I continue to believe that energetically useful amounts of oxygen are not going to be found in lifeless nature, precisely because they are energetic and would quickly react away.
I suppose there is the possibility that a planet could be so rich in oxygen that it would be stoichiometrically in excess of hydrogen, carbon, silicon, all metals, and a few other elements (S, P, etc.) combined. It would have to be all combined, because they all prefer oxygen to each other. Then, theoretically, at some point there would be nothing left to oxidize and O2 might occur in free form. However, it would likely be accompanied by lots of CO2, which would give it away.
In our solar system, the abundances of oxygen and carbon are such that there is more than enough carbon to bind all the oxygen, meaning O2 should be rare and CO2 common, even after ALL the hydrogen escapes. This is exactly what we see on all the (well, two only, I suppose…) big rocky planets other than Earth.
Perhaps some other stellar systems have a much higher O/C ratio. Then we might see abiotic oxygen atmospheres. We will likely know the O/C ratio, though, so we will not be that easily fooled.
@jkittle:
I wonder what leads you to believe that?
Myself, I tend towards the opposite opinion, that the sequestration of reduced carbon is the only way to get oxygen into the atmosphere. For this to happen, the carbon must be reduced first, despite the presence of an oxidizing atmosphere. This is an unfavorable, endothermic reaction. Only life would do such a thing, because it requires reduced carbon for its biomass.
Fascinating discovery! So, the presence of Oxygen may or may not depend on the existence of Life. It will only be the detection of Oxygen along side of other chemical species that would constitute a smoking gun detection of Life in the atmosphere of a planet.
Although this discovery certainly reminds us that there could be other processes that lead to oxygen in the atmosphere of planets and moons, what implication might it have for exploration and colonization? For example, if we find an exoplanet with reasonable temperatures, pressure, water, and organics but none of the other gases needed to clinch the case for Life, then perhaps we could settle such a breathable world without needing to terraform, without needing to disturb an existing ecology, and without needing to worry about biological incompatibility and/or toxicity. Monodian habitable planets of this sort would seem to be the most ideal worlds for eventual colonization and/or seeding missions for precisely the reasons just mentioned.
“It will only be the detection of Oxygen along side of other chemical species that would constitute a smoking gun…”
No combination of simple molecules is “smoking gun” for life. It can at most be suggestive for further investigation. There are too many possible combinations of abiotic chemistry, only a tiny fraction of which we have discovered.
The only unequivocal evidence of life we could gather telescopically would be spectroscopic evidence of molecules too complex to have formed abiotically (e.g. a functional equivalent of chlorophyll).